Over the past twenty years, computer technology has evolved very rapidly. One aspect of this evolution has been a progressively growing demand for increased storage capacity in memory devices. In order to provide a high storage density at a reasonable cost, one of the most enduring techniques has been to provide a rotatable hard disk with a layer of magnetic material thereon, and a read/write head which is supported for movement adjacent the disk and can transfer information to and from the disk.
Early disk drives included a read/write head having a single read/write element, which was used both for writing data and reading data. However, there has been a progressively increasing demand for disk drives with significantly higher storage densities, and one result is that new types of heads have come into common use, examples of which include the magneto-resistive (MR) head, and the giant magneto-resistive (GMR) head. These MR and GMR heads typically have one element for writing data and a separate element for reading data, and these read and write elements are physically spaced from each other.
As is known in the art, a head can be positioned with respect to a disk by using feedback control based on servo information read from the disk with a read element of the head. In a head with spaced read and write elements, the read element is used to position the head relative to the disk not only for reading, but also for writing. One aspect of this is that, as the head is moved relative to the disk, the orientation of the read and write elements varies with respect to the tracks on the disk, such that the write element is typically aligned with a track that is different from the track with which the read element is aligned. Consequently, in order to correctly position the write element over a selected track for the purpose of writing data to that track, the read element must be positioned at a location which is radially offset from the selected track. This radial offset is referred to as a “microjog”, and has a magnitude which varies as the head moves radially with respect to the disk. Techniques have been developed for calculating microjog values, and have been generally adequate for their intended purposes, but they have not been satisfactory in all respects.
As one aspect of this, there are existing disk drives in which the disk is rotatably supported in a removable cartridge, and in which the head is movably supported in a drive unit that can removably receive the cartridge. A given drive unit must be able to work with any of several similar and interchangeable cartridges, and any given cartridge must be capable of working in any of a number of compatible drive units. The removability of the cartridge introduces a number of real-world considerations into the system, and these considerations affect the accurate calculation of a microjog value.
For example, the cartridges have manufacturing tolerances which vary from cartridge. Thus, from cartridge to cartridge, there will be some variation relative to the cartridge housing of the exact position of the axis of rotation of the disk. As another example, two different cartridges may have slightly different mechanical seatings when they are inserted into the same drive unit. In fact, a given cartridge may experience different mechanical seatings on two successive insertions into the same drive unit. Real-world variations of this type cause small variations in the orientation of the read/write head with respect to the tracks on the disk, and thus affect accurate calculation of a microjog value.
In order to realize higher data storage densities in systems of the type which utilize removable cartridges, it is desirable to be able to use read/write heads that facilitate high storage densities, especially read/write heads that have spaced read and write elements, such as MR and GMR heads. However, due to real-world considerations of the type discussed above, accurate calculation of a microjog value has presented problems in the context of a removable cartridge. Accordingly, existing systems that use removable cartridges have continued to use read/write heads with a single read/write element, with the consequence that the storage capacities are significantly less than the storage capacities desired by consumers.
Current magnetic recording devices use an inductive-write and GMR-read dual element head (FIG. 1). Due to head manufacturing limitations, the read and write elements are not necessarily aligned along a centerline.
In addition, most hard drives place the head on a rotary actuator. When the actuator rotates around its pivot to position the head over a particular track, the GMR reader and inductive writer can be several servo tracks apart due to the finite distance between them (FIG. 2).
Hard drives minimize the complications caused by this head and actuator geometry by positioning the reader at a servo track center and writing data wherever the writer happens to be. Adjacent tracks are written in the same manner. If the servo tracks are evenly spaced there should always be a constant distance between the centerlines of the written data (FIG. 3). The hard drive disk(s) can not be removed, so the read and write elements will always be the same. Knowledge of the reader to writer spacing and offset is required to accurately place the reader over the written data upon read back.
A removable hard disk drive such as the REV drive from Iomega Corporation uses the same dual element head, but allows the user to remove the disk from the drive. This means the data written on a given disk can come from multiple REV drives. Each drive has its own set of heads, and the separation and offset between the reader and writer can vary from head to head. Using the hard drive technique described above, which places the reader at the track center and writing data wherever the writer is, will not work because of this drive to drive variation. To ensure all REV drives can read and write any cartridge, the REV drive requires the writer to be placed over the data track center. This implies the head geometry for each drive has to be pre-determined. During data writes, the GMR reader is positioned wherever necessary so the data is always written down the data track center. All REV drives expect to find the data in the center of the defined data track.
FIG. 4 illustrates a typical servo and data track layout. Servo tracks are defined by writing specific patterns to the disk which are never overwritten. The reader width is designed to be narrower (˜60%) than the writer. This allows the servo track pitch to be higher than the data track pitch. Ideally the servo tracks would be evenly spaced. Due to spindle motor runout, windage, vibration, temperature variations, etc., the servo tracks vary in width. Inaccuracies in the servo track widths can cause inaccuracies in positioning the GMR reader and inductive writer. Positioning inaccuracies ultimately cause two problems. The first is known as “data encroachment”. Data encroachment occurs when the writer is positioned away from the data track center and overwrites part of the adjacent track data. This is catastrophic if the adjacent track data is unrecoverable. The second issue occurs during data read back. Since the data is expected to be on the data track center, the servo system will position the reader there first. If problems occur reading the data, the servo system will re-position the head at varying off track locations in an attempt to find it. This process takes time which affects the overall data throughput.
As described above, accurately positioning the reader and writer is important to successfully storing data without data encroachment, and efficiently reading that data back. Varying servo track widths cause position errors in two different ways. The first involves inaccuracies in generating a linear position error signal from the servo information written on the disk within a given servo track. The second comes from the fact that the GMR reader can be several servo tracks away from the inductive writer. This is the cumulative effect of many servo track widths in error. Since the writer needs to be positioned over a data track center, the reader to writer distance at a particular disk radius must be pre-determined. Using the servo tracks as the measuring tool, any inaccuracy in servo track width directly translates to writer positioning errors. What is needed is an efficient manner to compensate for these errors.